The present invention is generally related to radio frequency filters, and more particularly to a transmitter filter with an integral directional coupler for use in radio signal transmitters such as cellular telephones.
In cellular telephones, it is necessary to accurately maintain the magnitude of the output power at one of eight different levels during cellular telephone calls. Such cellular telephones typically include power control circuitry of the type described in U.S. Pat. No. 4,523,155 for accurately maintaining the magnitude of the output power at the desired power level. The magnitude of the output power from the cellular telephone transmitter may be sampled by a capacitor, as illustrated in U.S. Pat. No. 4,523,155, or by a directional coupler. A directional coupler is preferable over a capacitor since it only detects power in the forward direction. Such directional couplers are typically implemented on printed circuit boards by means of adjacent, parallel coupled transmission lines, one line coupling the power amplifier to the transmitter filter and the second lien terminated on one end and coupled to detecting circuitry on the other end for producing an output signal having a magnitude proportional to the magnitude of the power amplifier output signal. However, directional couplers implemented on certain printed circuit boards are inadequate since their net insertion loss is no better than approximately 0.7 dB, their unloaded q is no better than approximately 40, and their characteristic impedance can vary by approximately 10% due to dimensional tolerances. For the foregoing reasons, there is a need for an improved transmitter filter with integral directional coupler for use in radio signal transmitters such as cellular telephones.